The fact that the door has no window strongly suggests that it is a painted Argosy. Most (but not all) Airstream branded MH's had windows in the door.
The clincher would be for you to take a magnet with you when you go see it. The Argosy units have galvanized steel front and rear caps. The Airstream units are all aluminum.
The 1976 units have two round headlights. The 1977 and later units have the 4 square headlights. My strong hunch is that it is a painted 1977-1979 Argosy. Brett won't be in Tampa until Sunday so you have until then to close the deal. He is collecting Argosy MH's!